pos/pos_multi_ean/models/product.py

import json
from odoo import models, fields, api


class Product(models.Model):
    _inherit = 'product.product'

    # technical field used in POS frontend
    multi_ean_json = fields.Char(
        "Multi EAN list", readonly=True,
        compute="_compute_multi_ean_json")

    @api.multi
    def _compute_multi_ean_json(self):
        for p in self:
            multi_ean_json = [x for x in p.mapped('ean13_ids.name') if x]
            p.multi_ean_json = json.dumps(multi_ean_json)